International Application Document No. WO 97/10115 discusses that one may adjust each sensor cell of a seat mat to a mechanical stress which is conditioned by the poor installation of the seat mat in a vehicle seat. Such a seat mat is used for determining whether there is a person on the vehicle seat, and what kind of sitting position this person has taken up, and also may be used to determine the weight of the person. These data are then used to activate an air bag.